Liquid level and temperature monitoring apparatus

ABSTRACT

An apparatus is provided for use in monitoring the level and temperature of a refrigerant, wherein the temperature of the liquid phase of the refrigerant is to be maintained below an established acceptable value. The apparatus includes a chamber interposed in a liquid line of a cooling system, wherein the chamber has a transverse cross-sectional area greater than the area of the liquid line, and is disposed at least partially above the line. A temperature sensor and a heater are provided within the chamber above the height of the liquid line and the sensor is connected to an alarm through a control which measures the temperature of the sensor and activates the alarm when the sensor temperature exceeds a predetermined value above the established acceptable value of the refrigerant liquid. The heater is of a temperature greater than the predetermined value so that when the liquid within the chamber falls below the temperature sensor and heater, heat is transferred to the sensor and raises the temperature of the sensor, and when the liquid level within the chamber reaches the sensor and heater, heat is transferred to the liquid and the temperature of the sensor is maintained at about the same value as the liquid.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to devices for monitoring thelevel and temperature of a liquid flowing through a fluid handlingsystem and, more particularly, to a monitoring apparatus for providingan alarm when the flow or temperature of a liquid within a refrigerationsystem falls outside an established acceptable range of values.

2. Discussion of the Prior Art

Refrigeration-cycle systems are used to provide cooling in everythingfrom residential air conditioners to commercial freezers such as thoseemployed to cool refrigerated or frozen food counters in grocery stores.

A conventional commercial refrigerator or freezer includes a condenser,an evaporator, liquid and suction lines connected between the condenserand the evaporator to form a closed coolant system, a compressor, and athermostatic expansion valve in the liquid line adjacent the evaporator.A liquid control valve is typically provided in the liquid line and ismovable between a closed, defrost-cycle position restricting the flow ofliquid through the liquid line, and an open, refrigeration-cycleposition.

It is a known practice to defrost a commercial refrigeration systemdaily, and frequently twice daily, in order to maintain the efficiencyof the system and prevent the formation of ice within the counter. Anelectrical circuit is provided in conventional systems for controllingthe defrost cycle, and includes a switch for permitting operation of theliquid control valve to switch the system between the defrost cycle andthe refrigeration cycle.

In order to permit maintenance personnel to monitor the level ofrefrigerant in the system, it is known to provide a sight glass withinthe liquid line which enables the personnel to view the flow of liquidthrough the line. When bubbles are visible in the line, low levels ofcoolant are present, providing an indication that replenishment isrequired. However, a certain amount of coolant is typically lost fromthe system before bubbles ever begin to appear in the sight glass,rendering early detection of coolant lose impossible.

This shortcoming in conventional systems has become significant inrecent years with the concern for protecting the ozone from depletiondue to the release of freon and other coolants to the atmosphere. Withinthe last four years, largely due to government imposed taxes, the priceof freon has increased by a factor of five, bringing to the forefront aneed to conserve freon in refrigeration systems whenever possible, andto stop leaks at an early stage.

New refrigeration systems are being marketed which include complicatedmonitoring systems for monitoring the level of liquid refrigerant in areceiver tank of the system, and which are capable of detecting certaintypes of leaks in the system. However, these systems are very expensive,and do not address the need of providing a reliable monitoring apparatuscapable of use in existing, older units.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the present invention to provide a liquid level andtemperature monitoring apparatus which can be retrofit for use in anexisting refrigeration system for alerting maintenance personnel that afailure of the system has occurred.

In accordance with this object, the present invention aims to provide anapparatus capable of sensing the lose of refrigerant from the system,the failure of a circulation fan or other condenser component, or thefailure of the system control unit used to switch the system between adefrost cycle and a refrigeration cycle.

Another object of the present invention is to provide a monitoringapparatus capable of alerting maintenance personnel when air is trappedwithin the system, and of permitting relief or evacuation of the air.

In accordance with these and other objects evident from the followingdescription of a preferred embodiment of the invention, an apparatus isprovided for use in monitoring the level and temperature of a liquidflowing through a horizontally extending length of line formed by firstand second line sections, wherein the temperature of the liquid is to bemaintained below an established acceptable value.

The apparatus includes a chamber interposed between the first and secondline sections, the chamber including a transverse cross-sectional areagreater than the transverse cross-sectional area of the line. Thechamber is disposed at least partially above the line and is normallyclosed except for an inlet and an outlet in fluid communication with theline.

A temperature sensor is provided within the chamber above the height ofthe line, as is a heater which is adjacent the temperature sensor andwhich is of a temperature greater than the predetermined value so thatwhen the liquid within the chamber falls below the temperature sensorand heater, heat is transferred to the sensor and raises the temperatureof the sensor, and when the liquid level within the chamber reaches thesensor and heater, heat is transferred to the liquid and the temperatureof the sensor is maintained at about the same value as the liquid. Acontrol means measures the temperature of the sensor and activates analarm when the sensor temperature exceeds a predetermined value abovethe established acceptable value.

By constructing a monitoring apparatus in accordance with the presentinvention, numerous advantages are obtained. For example, by providingboth a sensor and a heater within the chamber, and by controlling thepredetermined sensed temperature value at which the alarm sounds, it ispossible to detect low liquid levels within the line, such as thosecaused by the leakage of coolant from the system, by air being trappedwithin the chamber, or by the failure of the liquid solenoid used toswitch the system between defrost and refrigeration cycles.

Further, this construction provides an alarm when the temperature of theliquid in the line exceeds an acceptable value, and thus provides anindication that the condenser, or some component thereof has failed.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

A preferred embodiment of the present invention is described in detailbelow with references to the attached drawing figures, wherein:

FIG. 1 is a schematic view of a refrigeration system including amonitoring apparatus constructed in accordance with the preferredembodiment;

FIG. 2 is a circuit diagram of the monitoring apparatus, illustratingthe manner in which the apparatus is electrically connected within therefrigeration system;

FIG. 3 is a fragmentary side elevational view of the monitoringapparatus, illustrating the various components thereof;

FIG. 4 is a sectional view of an in-line chamber of the apparatus, takenalong 4--4 of the FIG. 3;

FIG. 5 is a sectional view of the chamber, taken along line 5--5 of FIG.4;

FIG. 6 is a sectional view of the chamber, taken along line 6--6 of FIG.4; and

FIG. 7 is a elevational view of a rheostat dial employed in thepreferred embodiment of the apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A refrigeration system is illustrated in FIG. 1 and includes a condenser10, an evaporator 12, liquid and suction lines 14, 16 connected betweenthe condenser and the evaporator, a compressor 18, and a motor 20 fordriving the compressor.

The evaporator 12 is provided within a counter or freezer to be cooled,and a thermostatic expansion valve 22 is provided in the liquid lineadjacent the evaporator. The suction line 16 extends from the counter orfreezer to the condenser 10, which is typically positioned on the roofof the building within which the refrigeration system is disposed.

Although not shown, the condenser 10 consists of a cooling tower and afan for drawing air through the tower to cool the refrigerant. Areceiver tank 24 is commonly provided within the building close to theevaporator 12 and is connected to the liquid line 14. A return line 26extends between the condenser 10 and the receiver tank 24 for deliveringliquid refrigerant to the tank.

A liquid line solenoid valve 28 is provided in the liquid line 14, andis movable between a closed, defrost-cycle position and an open,refrigeration-cycle position, and a restrictive bypass line 30 extendsaround the valve for allowing a fraction of the liquid refrigerant inthe receiver tank 24 to flow into the liquid line 14 during defrost.

The liquid line solenoid 28 is electrically actuated, and a circuit isprovided for controlling operation of the solenoid to switch the systembetween the defrost and refrigeration cycles. Preferably, therefrigerant is freon, which must be maintained within the liquid line ata temperature below an established acceptable value of between 100-200°F., and preferably below 120-124° F.

In addition to the foregoing components, a liquid dryer 32 may also beprovided within the liquid line, as may a sight glass 34 which permitsmaintenance personnel to visually inspect the flow of liquid coolantthrough the line 14.

The refrigeration system described thus far is conventional, and mayeither be constructed with a monitoring apparatus in accordance with theinvention, or may be retrofit with such a monitoring apparatus withoutdeparting from the scope of the present invention.

The monitoring apparatus is illustrated in FIG. 3, and generallyincludes a chamber 36 disposed in fluid communication with ahorizontally extending section of the liquid line 14, a cabinet 38mounted in proximity to the chamber, and a remote alarm 40 positionednear an operator responsible for maintenance of the system.

The chamber 36 includes a transverse cross-sectional area greater thanthe transverse cross-sectional area of the liquid line 14, and a majorportion of the chamber is disposed above the liquid line. The chamberalso includes an inlet 42 and an outlet 44, each in fluid communicationwith the liquid line 14. The chamber is otherwise air-tight so as topermit air to be completely removed from the system without leaking backinto the chamber.

Preferably, the chamber 36 is formed of a tubular side wall 46 having adiameter about four times greater than the diameter of the liquid line,and end walls 48, 50 secured to the side wall by welding or the like.The chamber is constructed of steel, copper or other equivalent materialcapable of containing the pressures commonly employed in refrigerationsystems, e.g. of about 500 psi.

Turning to FIG. 6, the inlet 42 and outlet 44 are formed in opposing endplates adjacent the lower edges thereof, and the liquid line sectionsconnected with the chamber extend into the chamber and are welded to theend plates to provide an air-tight seal between the chamber and the line14.

As illustrated in FIG. 5, the end plate 50 includes a second hole formedabove the outlet 44 for receiving a pair of elongated, tubular sleeves52, 54. Each of these sleeves extends through the opening in the endwall into the chamber, and includes an inner closed end 56, 58 withinthe chamber and an outer open end 60, 62 respectively. The interiorspaces within the sleeves are isolated from direct fluid contact withthe chamber so that fluid within the chamber is not allowed to leakthrough the sleeves.

The sleeves are formed of steel, copper, or any other suitable materialcapable of withstanding the noted pressures of the system, and arewelded to the end plate to provide an air-tight seal around the openingthrough which the sleeves extend.

As shown in FIG. 4, the sleeves are connected together, e.g. by welds 64or the like, so that heat within the sleeve 52 is conducted to thelarger sleeve 54. Preferably, the weld extends along the entire lengthof the sleeves within the chamber and is formed of a heat conductivematerial so that heat transfer between the sleeves is allowed.

Returning to FIG. 6, the side wall of the chamber is provided with avalve 66 on the top side thereof which permits selective fluidcommunication with the interior of the chamber. Preferably, the valve isa Schrader valve similar in design to valves used in automobile tiresand the like, and is operable to release air that is trapped within thepressurized chamber. Because freon is of a density greater than air, theair settles on top of the freon adjacent the valve 66.

As shown in FIG. 5, a temperature sensor 68 is provided within theelongated sleeve 54 and extends into the chamber so that when thechamber is full of liquid, the temperature of the sensor is maintainedat about the same temperature as the liquid. Preferably, the sensor 68is a capillary bulb, and includes a capillary tube 70 extending betweenthe bulb and a thermostat 72, illustrated in FIG. 2.

The thermostat 72 is of known construction and functions as a controlmeans for measuring the temperature of the sensor and activating thealarm when the sensor temperature exceeds a predetermined value above anestablished acceptable value stored in the thermostat. Preferably, thethermostat 72 includes an adjustment control dial allowing adjustment ofthe predetermined value within a range of from about 100° F. to about200° F. An ideal setting for the predetermined temperature value is120-124° F.

As shown in FIG. 5, a heater 74 is provided in the smaller sleeve 52 andextends into the interior of the chamber adjacent the temperature sensor68. The heater is an electrical resistance heater, preferably of thepencil type, and is elongated so that heat generated within the heateris transferred to the sleeve 52 and conducted through the welds 64 andsleeve 54 to the sensor 68. By providing this construction, when theliquid within the chamber falls below the sleeves 52, 54 housing thetemperature sensor 68 and heater 74, heat reaches the sensor and raisesthe temperature of the sensor, and when the liquid within the chambercontacts the sleeves, heat is drawn from the sleeves 52, 54 to theliquid and the temperature of the sensor is maintained at about the samevalue as the liquid.

Returning to FIG. 2, the heater 74 is connected to a power source 76through a variable resister 78, e.g. a rheostat or the like, whichpermits adjustment of the output of the heater. Thus, a delay adjustmentmeans is defined for varying the amount of heat generated by the heaterso that the period of delay between the time at which the liquid levelin the chamber falls beneath the sleeves 52, 54, and the time at whichthe temperature of the sensor is raised to the predetermined value maybe adjusted.

Preferably, the rheostat 78 includes an on/off position, as shown inFIG. 7, and also includes a number of settings displayed in minutes,representative of the period of delay obtained at each setting.

The alarm 40 is an audible indicator, and preferably includes a bellcapable of being positioned remote from the chamber. An electricalconnection 80 is provided between the bell and the thermostat 72 forpermitting the bell to be energized when the sensor reaches thepredetermined value.

The circuit employed in the apparatus is illustrated in FIG. 2, andincludes the power source 76 which may either be connected directly tothe apparatus, or which may be connected through the liquid linesolenoid 28 of the refrigeration system. The heater 74 and rheostat 78receive power directly from the source and are arranged in series sothat adjustment of the rheostat varies the voltage to the heater. Astep-down transformer 8 is provided between the power source 76 and thethermostat 72 for reducing the voltage to the thermostat, and to thealarm 40 which is arranged in series with the thermostat.

The rheostat 78, thermostat 72 and transformer 82 are housed within thecabinet 38, shown in FIG. 3, and the cabinet preferably includes a covermovable between a closed position preventing access to these components,and an open position allowing access thereto. The cover includes a lock86 for preventing the cabinet from being opened by unauthorizedpersonnel. Although the cover is preferably formed of metal, any othermaterial may be used which provides suitable protection of the enclosedcomponents from meddling and from harmful ambient conditions.

With reference to FIG. 2, during normal operation in a refrigerationcycle, a switch 86 within the liquid line solenoid circuit is closed sothat the solenoid 28 is energized and retained in an open positionallowing liquid refrigerant within the receiver tank 24, shown in FIG.3, to flow into the liquid line 14 under pressure. Closure of the switch86 also energizes the monitoring apparatus so that heat is generated bythe heater 74, as controlled by the rheostat 78, and the thermostat 72is activated to energize the alarm 40 when the temperature of the sensor68 exceeds the predetermined value stored by the thermostat.

As long as the liquid line 14 is fully pressurized and the system isfull of refrigerant, the liquid within the chamber contacts the sleeves52, 54, shown in FIG. 4, and draws the heat from the heater 74 before itis transferred to the sensor 68.

During defrost, the switch 86 in the liquid line solenoid circuit 88,shown in FIG. 2, is opened, causing de-energization of the solenoidvalve 28 and deactivation of the monitoring apparatus. By providing thisconstruction, it is possible to defrost the system, which can take from30-60 minutes, without setting off the alarm. If the apparatus were notdeactivated, the reduced flow through the bypass line 30, as shown inFIG. 1, would be insufficient to fill the chamber, and after the presetdelay, the heater would raise the temperature of the sensor to thepredetermined value and the alarm would sound.

If a leak of the refrigerant exists, the level of liquid within thechamber drops, falling beneath the sleeves 52, 54. Thereafter, heatgenerated by the heater is conducted through the sleeves and is allowedto radiate within the sleeve 54 so as to heat the sensor 68. Because theliquid in the chamber is not available for drawing heat away from thesleeves, heat continues to be transferred until the sensor temperatureapproaches that of the heater. Because the heater is maintained at atemperature greater than the predetermined value stored in thethermostat, a low liquid level will result in activation of the alarm ifthe low level condition continues for a sufficient length of time.

This length of time is adjustable by properly setting the rheostat 78 tocontrol the heat generated by the heater. If voltage to the heater isreduced the delay time is increased, but if the voltage is increased,less of a delay occurs. For example, in an exemplary system, a delay of60 minutes is experienced when 30 Volts are supplied to the heater, adelay of 30 minutes is experienced when 40 Volts are supplied to theheater, and a delay of 10 minutes is experienced when 60 Volts aresupplied.

If the liquid level in the chamber drops temporarily, but rises again toa level covering the sleeves prior to activation of the alarm, heat fromthe sleeves is transferred again to the liquid and the alarm will notsound. Thus temporary fluctuations in the liquid level do not causealarm unless low level condition exceeds the delay period. A delay of 30minutes is preferred because it prevents frequent "false" alarms thatwould be caused by normal system fluctuations, but still detects a leakbefore a significant amount of refrigerant is lost.

If there is a cooling tower failure, such as a busted fan or the like,and the liquid delivered to the receiver tank 24 is not properly cooled,the liquid level within the chamber 36, although maybe continuing tocontact the sleeves 52, 54, will be of a temperature higher than normal.If this temperature exceeds the predetermined value stored in thethermostat, heat from the liquid will raise the temperature of thesensor, causing activation of the alarm.

If air gets into the system, which is normally evacuated, the chamberprovides a trap from which the air can not escape. As mentioned, becausethe air is lighter than the refrigerant, it is trapped adjacent thevalve 66 on the upper surface of the chamber and forces the liquid leveldownward. If sufficient air is trapped in the chamber, the sleeves areuncovered allowing the heater to raise the temperature of the sensor tothe predetermined value, causing the alarm to sound.

In addition to providing an alarm in each of the foregoingcircumstances, the monitoring apparatus will also sound when any of anumber of additional problems arise which have the effect of eitherreducing the pressure in the liquid line or of raising the temperatureof the refrigerant in the liquid line. Thus, maintenance personnelalerted by the alarm are able to diagnosis the problem causing the alarmby systematically checking the various conditions capable of causingthese conditions within the chamber.

Although the invention has been described with reference to thepreferred embodiment illustrated in the attached drawing figures, it isnoted that substitutions may be made and equivalents employed hereinwithout departing from the scope of the invention as recited in theclaims.

What is claimed is:
 1. An apparatus for use in monitoring the level and temperature of a liquid flowing through a horizontally extending line formed by first and second line sections, wherein the temperature of the liquid is to be maintained below an established acceptable value, the liquid level and temperature monitoring apparatus comprising:a chamber interposed between the first and second line sections and including a transverse cross-sectional area greater than the traverse cross-sectional area of the liquid line, the chamber being disposed at least partially above the line and being normally closed except for an inlet in fluid communication with the first line section and an outlet in fluid communication with the second line section; a temperature sensor provided within the chamber above the height of the line; an alarm; a control means for measuring the temperature of the sensor and activating the alarm when the sensor temperature exceeds a predetermined value above the established acceptable value; a heater provided within the chamber adjacent the temperature sensor above the height of the line, the heater being of a temperature greater than the predetermined value so that when the liquid within the chamber falls below the temperature sensor and heater, heat is transferred to the sensor and raises the temperature of the sensor, and when the liquid level within the chamber reaches the sensor and heater, heat is transferred to the liquid and the temperature of the sensor is maintained at about the same value as the liquid; and a casing within which the control means is housed, the casing a cover movable between a closed position protecting the control means and an open position exposing the control means, and a lock for locking the cover in the closed position, wherein the casing at least partially protects the control means from ambient conditions.
 2. An apparatus for use in monitoring the level and temperature of a liquid flowing through a horizontally extending line formed by first and second line sections, wherein the temperature of the liquid is to be maintained below an established acceptable value, the liquid level and temperature monitoring apparatus comprising:chamber interposed between the first and second line sections and including a transverse cross-sectional area greater than the transverse cross-sectional area of the liquid line, the chamber being disposed at least partially above the line and being normally closed except for an inlet in fluid communication with the first line section and an outlet in fluid communication with the second line section, the chamber including an upper wall within which a valve is provided for selectively venting gas from within the chamber; a temperature sensor provided within the chamber above the height of the line; an alarm; a control means for measuring the temperature of the sensor and activating the alarm when the sensor temperature exceeds a predetermined value above the established acceptable value; and a heater provided within the chamber adjacent the temperature sensor above the height of the line, the heater being of a temperature greater than the predetermined value so that when the liquid within the chamber falls below the temperature sensor and heater, heat is transferred to the sensor and raises the temperature of the sensor, and when the liquid level within the chamber reaches the sensor and heater, heat is transferred to the liquid and the temperature of the sensor is maintained at about the same value as the liquid.
 3. An apparatus for use in monitoring the level and temperature of a liquid flowing through a horizontally extending line formed by first and second line sections, wherein the temperature of the liquid is to be maintained below an established acceptable value, the liquid level and temperature monitoring apparatus comprising:a chamber interposed between the first and second line sections and including a transverse cross-sectional area greater than the transverse cross-sectional area of the liquid line, the chamber being disposed at least partially above the line and being normally closed except for an inlet in fluid communication with the first line section and an outlet in fluid communication with the second line section, the chamber being defined by a tubular side wall and a pair of circular end walls, the inlet and outlet being formed in the end walls, the chamber including a pair of elongated sleeves extending through one of the end walls into the chamber, each sleeve being tubular and having an inner closed end within the chamber and an outer open end to define interior sleeve spaces which are isolated from direct fluid contact with the chamber; a temperature sensor provided within the chamber above the height of the line; an alarm; a control means for measuring the temperature of the sensor and activating the alarm when the sensor temperature exceeds a predetermined value above the established acceptable value; and a heater provided within the chamber adjacent the temperature sensor above the height of the line, the heater being of a temperature greater than the predetermined value so that when the liquid within the chamber falls below the temperature sensor and heater, heat is transferred to the sensor and raises the temperature of the sensor, and when the liquid level within the chamber reaches the sensor and heater, heat is transferred to the liquid and the temperature of the sensor is maintained at about the same value as the liquid.
 4. The apparatus as recited in claim 3, wherein the sleeves are arranged side-by-side and are connected together by a heat conductive material.
 5. The apparatus as recited in claim 4, wherein the sensor is positioned within one of the sleeves and the heater is provided within the other sleeve so that when the chamber is filled with liquid heat is transferred from the heater and the sleeves to the liquid, and when the liquid in the chamber is below the sleeves, heat from the heater is transferred through the sleeves to the sensor.
 6. An apparatus for use in monitoring the level and temperature of a liquid flowing through a horizontally extending line formed by first and second line sections, wherein the temperature of the liquid is to be maintained below an established apparatus comprising:a chamber interposed between the first and second line sections and including a transverse cross-sectional area greater than the transverse cross-sectional area of the liquid line, the chamber being disposed at least partially above the line and being normally closed except for an inlet in fluid communication with the first line section and an outlet in fluid communication with the second line section; a temperature sensor provided within the chamber above the height of the line; an alarm; a control means for measuring the temperature of the sensor and activating the alarm when the sensor temperature exceeds a predetermined value above the established acceptable value; a heater provided within the chamber adjacent the temperature sensor above the height of the line, the heater being of a temperature greater than the predetermined value so that when the liquid within the chamber falls below the temperature sensor and heater, heat is transferred to the sensor and raises the temperature of the sensor, and when the liquid level within the chamber reaches the sensor and heater, heat is transferred to the liquid and the temperature of the sensor is maintained at about the same value as the liquid; and delay adjustment means for varying the amount of heat generated by the heater so that the period of delay between the time at which the liquid level in the chamber falls beneath the sensor and heater, and the time at which the temperature of the sensor is raised to the predetermined value may be adjusted.
 7. The apparatus as recited in claim 6, wherein the alarm generates an audible signal.
 8. The apparatus as recited in claim 6, wherein the control means includes a setting means for permitting adjustment of the predetermined value.
 9. The apparatus as recited in claim 8, wherein the predetermined value may be adjusted within the range of 100-200° F.
 10. An apparatus as recited in claim 6, wherein the heater is an electrical resistance heater.
 11. The apparatus as recited in claim 6, wherein the delay adjustment means includes a rheostat, and the heater is an electrical resistance heater.
 12. The apparatus as recited in claim 6, wherein the delay adjustment means allows adjustment of the period of delay between 30 and 60 minutes.
 13. An apparatus for use in monitoring the level and temperature of a liquid flowing through a horizontally extending line formed by first and second line sections, wherein the temperature of the liquid is to be maintained below an established acceptable value, the liquid level and temperature monitoring apparatus comprising:a chamber interposed between the first and second line sections and including a transverse cross-sectional area greater than the transverse cross-sectional area of the liquid line, the chamber being disposed at least partially above the line and being normally closed except for an inlet in fluid communication with the first line section and an outlet in fluid communication with the second line section; a temperature sensor provided within the chamber above the height of the line; an alarm; a control means for measuring the temperature of the sensor and activating the alarm when the sensor temperature exceeds a predetermined value between 120-124° F.; and a heater provided within the chamber adjacent the temperature sensor above the height of the line, the heater being of a temperature greater than the predetermined value so that when the liquid within the chamber falls below the temperature sensor and heater, heat is transferred to the sensor and raises the temperature of the sensor, and when the liquid level within the chamber reaches the sensor and heater, heat is transferred to the liquid and the temperature of the sensor is maintained at about the same value as the liquid.
 14. In a cooling system including a condenser, an evaporator, liquid and suction lines connected between the condenser and the evaporator, wherein the liquid line includes at least one horizontal segment, a compressor, a thermostatic expansion valve in the liquid line adjacent the evaporator, a liquid control valve in the liquid line movable between a closed, defrost-cycle position and an open, refrigeration-cycle position, and an electrical circuit including a switch and the liquid control valve for permitting operation of the liquid control valve to switch the system between a defrost cycle and a refrigeration cycle, wherein a refrigerant is provided which must be maintained within the liquid line at a temperature below an established acceptable value, a liquid level and temperature monitoring apparatus comprising:a chamber interposed within the liquid line along the horizontal segment and including a transverse cross-sectional area greater than the transverse cross-sectional area of the liquid line, the chamber being disposed at least partially above the liquid line and being normally closed except for an inlet and an outlet each in fluid communication with the liquid line; a temperature sensor provided within the chamber above the height of the liquid line; an alarm; a control means for measuring the temperature of the sensor and activating the alarm when the sensor temperature exceeds a predetermined value above the established acceptable value; a heater provided within the chamber adjacent the temperature sensor above the height of the liquid line, the heater being of a temperature greater than the predetermined value so that when the liquid within the chamber is below the temperature sensor and heater, heat is transferred to the sensor and raises the temperature of the sensor, and when the liquid level within the chamber reaches the sensor and heater, heat is transferred to the liquid and the temperature of the sensor is maintained at about the same value as the liquid; and a circuit means for connecting the liquid level and temperature monitoring apparatus to the electrical circuit including the switch and liquid control valve for permitting operation of the apparatus only during a refrigeration cycle.
 15. An apparatus for use in monitoring the level and temperature of a liquid flowing through a horizontally extending line formed by first and second line sections, wherein the line is to remain full of liquid and the temperature of the liquid is to be maintained below an established acceptable value, the liquid level and temperature monitoring apparatus comprising:a chamber interposed between the first and second line sections and including a transverse cross-sectional area greater than the transverse cross-sectional area of the liquid line, the chamber being disposed at least partially above the line and being normally closed except for an inlet in fluid communication with the first line section and an outlet in fluid communication with the second line section; a sensing means provided within the chamber above the height of the line for sensing the level and temperature of the liquid flowing through the chamber; an alarm; a control means for measuring the liquid level and temperature sensed by the sensing means and for activating the alarm when either the level of the liquid in the chamber drops below a predetermined level or the temperature exceeds a predetermined value above the established acceptable value; and an air removal means disposed within the chamber above the height of the line for relieving trapped air from the chamber so that the trapped air does not force the liquid level below the sensing means.
 16. The apparatus as recited in claim 15, wherein the air removal means includes a valve for permitting selective fluid communication with the chamber for releasing air that is trapped within the chamber above the height of the line. 